Radio frequency (RF) switching circuitry is used throughout modern wireless communications devices. For example, RF switching circuitry may be used in antenna tuning circuitry, RF front end switching circuitry (e.g., antenna swapping switching circuitry, signal routing circuitry), and the like. Generally, RF switching circuitry may be used to route RF signals between one or more components and one or more antennas. Further, RF switching circuitry may be used to present a desired impedance or voltage to a particular node. Conventionally, the design of RF switching circuitry has been focused on providing adequate power handling capability with minimal insertion loss and non-linearity. As wireless communications standards continue to evolve, switching speed has become a primary design concern in addition to insertion loss, power handling capability, and linearity.
FIG. 1 is a functional schematic illustrating conventional RF switching circuitry 10. The conventional RF switching circuitry 10 includes a switch input node 12, a switch output node 14, and a control signal input node 16. A number of RF switching elements Q1-QN are coupled in series between the switch input node 12 and the switch output node 14. As shown in FIG. 1, each one of the RF switching elements Q1-QN is a field-effect transistor (FET) including a gate contact (G), a drain contact (D), and a source contact (S). A drain contact (D) of a first one of the RF switching elements Q1 is coupled to the switch input node 12, a source contact (S) of a last one of the RF switching elements QN is coupled to the switch output node 14, and the remaining RF switching elements Q1-QN are coupled drain contact (D) to source contact (S) as illustrated.
A number of parallel resistors R_P1-R_PN are each coupled between a drain contact (D) and a source contact (S) of a different one of the RF switching elements Q1-QN. Specifically, a first parallel resistor R_P1 is coupled between the drain contact (D) and the source contact (S) of the first one of the RF switching elements Q1, a second parallel resistor R_P2 is coupled between the drain contact (D) and the source contact (S) of a second one of the RF switching elements Q2, a third parallel resistor R_P3 is coupled between the drain contact (D) and the source contact (S) of a third one of the RF switching elements Q3, and a last parallel resistor R_PN is coupled between the drain contact (D) and the source contact (S) of the last one of the RF switching elements QN.
A number of gate resistors R_G1-R_GN are each coupled between a gate contact (G) of a different one of the RF switching elements Q1-QN and a common node 18. Specifically, a first gate resistor R_G1 is coupled between the gate contact (G) of the first one of the RF switching elements Q1 and the common node 18, a second gate resistor R_G2 is coupled between the gate contact (G) of the second one of the RF switching elements Q2 and the common node 18, a third gate resistor R_G3 is coupled between the gate contact (G) of the third one of the RF switching elements Q3 and the common node 18, and a last gate resistor R_GN is coupled between the last one of the RF switching elements QN and the common node 18. A common resistor R_C is coupled between the control signal input node 16 and the common node 18.
The RF switching elements Q1-QN may be enhancement type FETs, and accordingly may present a very high impedance when a control signal CNT that does not exceed a threshold voltage of each one of the RF switching elements Q1-QN at the gate contacts (G) thereof is provided. In other words, the RF switching elements Q1-QN may provide an open circuit (i.e., off state) in response to an inadequate control signal CNT. When an adequate control signal CNT is provided to the control signal input node 16, it passes through the common resistor R_C, and is distributed by the gate resistors R_G1-R_GN to the gate contact (G) of each one of the RF switching elements Q1-QN. This causes the RF switching elements Q1-QN to present a very low impedance, thereby connecting the switch input node 12 to the switch output node 14. In other words, the RF switching elements Q1-QN may provide a closed circuit (i.e., on state) in response to an adequate control signal CNT.
As will be appreciated by those skilled in the art, capacitances between the gate contact (G) and the drain contact (D) (i.e., gate-to-drain capacitance) and the gate contact (G) and the source contact (S) (i.e., gate-to-source capacitance) of each one of the RF switching elements Q1-QN may allow a small amount of current from RF signals passing through the conventional RF switching circuitry 10 to flow into the gate resistors R_G1-R_GN and the common resistor R_C. This leakage current is dissipated by the gate resistors R_G1-R_GN and the common resistor R_C, thereby resulting in resistive losses that reduce the quality of the RF signals passed through the conventional RF switching circuitry 10. As the power handling requirements of the conventional RF switching circuitry 10 increase, it may be necessary to add additional RF switching elements to avoid breakdown. These additional RF switching elements may add insertion loss, which is compensated for by increasing the size of the RF switching elements. However, larger RF switching elements also provide larger gate-to-drain and gate-to-source capacitances, which increase the leakage current and thus resistive losses discussed above. Conventionally, these leakage currents have been reduced by maximizing the total resistance of the gate resistors R_G1-R_GN and the common resistor R_C.
Maximizing the total resistance of the gate resistors R_G1-R_GN and the common resistor R_C often reduces the switching speed of the conventional RF switching circuitry 10. As will be appreciated by those skilled in the art, the switching time of the conventional RF switching circuitry 10 is proportional to the time constant τ=RC thereof. Accordingly, as the total resistance of the gate resistors R_G1-R_GN and the common resistor R_C increases, so does the switching time of the conventional RF switching circuitry. A balance must therefore be struck between reducing resistive losses by preventing current flow in the control signal path and minimizing the switching time of the conventional RF switching circuitry 10. Such balancing often results in sub-optimal RF switching circuitry, especially when considering the constraints of modern wireless communications devices that require faster and faster switching times to comply with evolving wireless communications standards.
Accordingly, there is a need for improved RF switching circuitry with reduced switching times and insertion loss.